I like to use the CATS method:

C - Content. What is the game/campaign going to be about (in general). What are the themes that you want to touch on?

A - Aim. What are the players trying to accomplish? How will you know when you've reached a satisfactory conclusion?

T - Tone. What is the overall tone of the game? Is it serious or light-hearted? Gritty or gonzo?

S - Safety. How will you make sure that everyone has a good time, and that you avoid re-traumatizing content? What topics should be avoided altogether or treated gingerly (Lines and Veils). How will players signal that you need to stop the game and resolve an issue if they feel uncomfortable to express that verbally (X-Card/Script Change.)

The goal is to have a consensus on all these topics.

I found The Lazy DM's guide quite useful. It has a lot of advice on various parts of running campaigns, including session 0. It assumes you'll run 5e, but I've found it useful even after I switched away from dnd. I can share the pdf with you if you'd like, send me a PM if so.

I would use the pre-written modules for mothership, there are a lot of really good ones.

For investigation horror I would read over some of the alexandrian articles on clues, and general advice: https://thealexandrian.net/wordpress/1118/roleplaying-games/three-clue-rule

Horror also is helped by pacing, don't be afraid to let silence sit and let tension grow but also make sure that it is building and not just the players standing around discussing.

I would highly reccommend doing: Mothership - Haunting of Ypsilon 14 as your oneshot it is fantastic. For longer term play I recommend: https://www.tuesdayknightgames.com/products/gradient-descent

For any PC that wants to horny larp, they get 2 points of exhaustion.